The invention relates to a color display device.
Such a device is known from PCT-application no. WO 99/34392-A1. The known device comprises a color cathode ray tube with an in-line electron gun for generating three electron beams, a color selection electrode and a phosphor screen on an inner surface of a display window, and a deflection unit for deflecting the electron beams across a color selection electrode. The known device also comprises a pair of means, which are arranged at some distance from each other and are intended to dynamically influence the trajectories of the electron beams so as to decrease the distance between the electron beams at the location of the deflection plane as a function of the deflection. By doing so, the distance between the electron beams decreases for larger deflection angles. The distance between the color selection electrode and the phosphor screen is inversely dependent on the distance between the electron beams. Thus a decrease of the distance between the electron beams enables the distance between the color selection electrode and the phosphor screen to be increased. This makes it possible to combine a flat or almost flat display screen with a color selection electrode having a larger curvature. Said larger curvature increases the strength of the color selection electrode and reduces doming and microphonics. The color selection electrode may be a shadow mask.
Although the known device operates satisfactorily, problems remain. Power consumption of the pair of co-operating means poses a problem, more particularly the power dissipation in the drive circuit(s) for the co-operating means.
It is an object of the invention to provide a color display device of the opening paragraph with reduced power needs.
To this end, an aspect of the invention provides a color displays device as defined in claim 1.
The co-operating means are not directly driven by the rectified vertical deflection current as shown in FIG. 5 of WO 99/34392-A1 (see also FIG. 3 of this application). Such direct driving has the disadvantage that the co-operating; means draw their power directly from the deflection circuit and are coupled directly to the deflection circuit. The current through the co-operating means cannot be more than the current through the deflection circuit, putting a limit on the available current through the co-operating means and also on the shape and form of the current. If the current through the co-operating means is to be smaller than the deflection current, a part of the deflection current will have to be bypassed, inevitably leading to power loss. Furthermore, differences in temperature between the co-operating means and the bypass will lead to fluctuations in the current through the co-operating means, resulting in fluctuations and temperature instability.
In the color display device in accordance with the invention, the color display device is provided with driving means having an input for a signal corresponding to the deflection current. The deflection current itself does not directly drive the co-operating means, eliminating or at least strongly reducing the problems described above. A signal corresponding to the deflection current could be, for instance, a voltage across a resistive or capacitive element through which the deflection current or a part of the deflection current runs.
The two co-operating means are arranged in series. As was found by the inventors, the supply voltage commonly used in sets for functioning of the co-operating means is high in relation to the impedance of the co-operating means. The current through the means is also rather high. As a result, when the two co-operating means are driven separately or in parallel, much power is dissipated in the driving circuit for the co-operating means.
By putting the two co-operating means in series, the power need for the cooperating means is reduced, and particularly the drive voltage for the co-operating means can be made to match the supply voltage for the driving circuit.
However, when the two co-operating means are arranged in series, problems arise due to the fact that, although the two co-operating means provide magnetic fields of similar strength and form and are often very similar or even equivalent in design, they can best be supplied with different currents. By supplying the difference to a node (connection point) in between the two co-operating means, the operation of the co-operating means is greatly improved without increasing the power requirements to a large degree.
These and other objects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.